The primary objective of this research is to understand the molecular mechanism underlying F-exclusion of T7 and related bacteriophages. This is a model system or host-parasite interactions where the genes involved can be easily manipulated by genetic approaches. Knowledge accrued in this particular exclusion system is being applied to others to determine the generality of the model. Classical and molecular genetic techniques are employed to investigate the pleiotropic physiological dysfunctions associated with F-exclusion. These include perturbations in gene expression, inhibition of DNA replication, and membrane defects. It will be determined if the primary block to phage development is the transcriptionally-mediated entry of phage DNA into the cell. How the phage proteins that are the targets of the exclusion system interact with E. coli DNA gyrase in normal phage development is to be investigated. A genetic selection is described for mutants altered in those host products that are involved in the exclusion system. These mutants should help define the site of interaction of the phage- and plasmid-coded genes and provide information on the primary cellular event that causes the arrest in phage development.